The murine teratocarcinoma system has become an important model of neoplastic differentiation. The undifferentiated stem cells, termed embryonal carcinoma (EC), are highly malignant, but the differentiated progeny are, with rare exception, benign. The EC stem cells can be induced to differentiate under the influence of several chemical agents, the most potent being retinoic acid (RA). Embryonal carcinoma tumor differentiation can be induced in vivo with a combination of agents resulting in prolonged survival of tumor-bearing hosts and, in some cases, cure of the malignancy by complete conversion to benign cells. The process can be studied in vitro, but at present little is known about mechanisms of action of inducing agents due in part to a lack of a suitable cell system and to lack of methods to quantitate differentiation. We will develop a rapid, reproducible method of quantitating EC differentiation and to analyze differentiation quantitatively in an EC cell system highly sensitive to chemical induction. Recently, I have discovered a way to establish "committment" to differentiation of EC cells by a 15-minute pulse of retinoic acid. This "committed" state is labile for up to 16\hours after the pulse of drug and can be interrupted by inhibitors for protein synthesis. After 24\hours, the "commitment" is not reversible although the cells are not as yet morphologically differentiated. Early protein synthesis appears to be an obligate requirement of retinoic acid-induced differentiation. In separate but related studies, we will test the chemical induction effective in the murine system on human EC cells of new and established cell lines. These studies could provide a necessary link from the murine model system to the application of differentiation-induction therapy for clinical, human teratocarcinomas.